Stabilization of microwave oscillations



g- 1955 E. NORTON 2,714,663

STABILIZATION OF MICROWAVE OS CILLATIONS Filed May 29, 1950 10- am [Wm/64 fr I- g 72 I I I I I Z LEL/ 11/ 4 1L WA I/U 1.1 57! I u W 12-22 3, M W mi/fl INVENTOR LUWELL E. Nnmmm fiamm 2,7l4,663 Patented Aug. 2, 1955 2,714,663 STABILHZATEUN F MICROWAVE OSCILLATIONS Lowell E. Norton, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 29, 1950, Serial No. 164,978 18 Claims. (Cl. 250-36) This invention relates to methods and systems for stabilizing the frequency of microwave oscillations as generated, for example, .by klystrons, magnetrons or other microwave generators.

In accordance with the present invention, a confined body of a gas, such as ammonia, which exhibits sharp molecular absorption at one or more microwave frequencies is subjected to a magnetic field to split an absorption line of the gas into a pair of Zeeman lines somewhat higher and lower than the desired frequency of the microwave oscillations. Before impression upon the gas, the oscillations are modulated to produce upper and lower sidebands which in frequency are normally respectively adjacent the Zeeman absorption lines. The sidebands, with or without the carrier, are impressed upon the gas and modulation applied either to the Zeeman fields or to the sidebands to effect sweeping of frequency ranges respectively above and below the de sired carrier frequency. The microwave energies trans mitted by the gas in the aforesaid frequency ranges are selectively demodulated to produce pairs of pulses which contain frequency-error information both in their relative amplitude and in their phase relationship.

Preferably, and further in accordance with the invention, the pairs of pulses are impressed upon a detector network which compares their phase or amplitude to produce an error-voltage of sense corresponding with the frequency-deviation of the oscillations and which, for automatic stabilization, may be applied to a frequencycontrol electrode of the oscillator tube or of a control tube associated therewith.

The invention further resides in methods and systems having the features of novelty and utility hereinafter described and claimed.

For a more detailed understanding of the invention, reference is made to the accompanying drawings, in which:

Fig. 1 schematically illustrates system;

Figs. 2a, 2b, 3a and 3b are explanatory figures referred to in discussion of other figures including Fig. 1;

Fig. 4 illustrates a modification of Fig. 1 using an amplitude-comparator for automatic stabilization; and

Fig. 5 illustrates another modification of Fig. 1 utilizing a phase-comparator for automatic stabilization.

Referring to Fig. 1, the generator of microwave oscillations may be a klystron, magnetron or other microwave generator, or it may be a lower frequency oscillator followed by a suitable number of harmonic amplifiers. In either event, the microwave energy is transmitted as by a waveguide, concentric line or other suitable transmission line 11 to a chamber 12 containing ammonia or other gas exhibiting sharp molecular absorption at a frequency f0 corresponding with the desired output frequency of oscillator 10. By way of example, frequency f may be 23,870.l megacycles, the frequency of the 3,3 line of ammonia. In copending applications, including Serial No. 1,240, various gases, including ammonia, ex-

a frequency control hibiting sharp molecular resonance or absorption at many microwave frequencies are identified. The chamber 12-, which may be a length of waveguide, is provided with windows l3, 13 of quartz, mica, or like material, serving as gas-tight seals which are substantially transparent to the microwaves.

The fine line absorptions of certain materials including ammonia are split when sub ected to a properly oriented magnetic field; splitting is known as the Zeeman effect. In Fig. 1, such splitting of the absorption line at frequency ft) is produced by the magnetic field of a coil frequency (Figs. 2a, 2b, 3a and 3b). The extent of splitting or the frequency difference between the lines L1. and Lu is determined by the intensity of the magnetic field of coil 16 which can be selected by adjustment of the current from source 17 or the setting of rheostat 18, or equivalent.

Prior to its impression upon the gas in cell 12, the icrowave energy is modulated at a lower frequency fm to produce sidebands L and U (Figs. 2a, 2b, 3a, frequencies (fkifm) which are respectively higher and lower than the carrier frequency of the microwave oscillations and which are respectively adjacent the Zeeman line frequencies (foifn) when there is null deviation from the desired frequency f0. In the particular arrangement shown in Fig. l, the sidebands are produced by im ressing upon a mixer 15 the outputs of the microwave oscillator 10 and oscillator 14 generating oscillations at a lower frequency fm: alternatively the sidebands may be produced by amplitude-modulation of the generated oscillations in manner well known per se. By way of example, frequency fm may be of the order of .5 megacycles.

Depending upon the chosen intensity of the Zeeman field and the chosen frequency of oscillator 14, or equivalent amplitude-modulator for oscillator lit, the sideband frequencies LL, Ln may be located either be tween the Zeeman lines (Figs. 2a and 3b) or beyond them (Figs. 2b and 3a). As previously above stated, low-frequency modulation may be applied either to the Zceman fields or to the sidebands to effect sweeping of frequency ranges above and below the desired frequency of the microwave oscillations. Selective absorption of microwave energy by the gas occurs each time either sideband sweeps or is swept by the corresponding Zeeman absorption line. There are thus four specifically different though generically similar methods of operation respectively illustrated by Figs. 2a, 2b, 3a and 3b and in turn later discussed.

Reverting to Fig. l, for discussion of generic aspects of the methods and systems comprehended by the invention: beyond the gas cell 12, the transmission line if is divided into two paths or channels (A, B), respectively including filters 21A, 215 each passing the microwave energy in one of the aforesaid swept frequency ranges and excluding the microwave energy within the other swept frequency range. Specifically, the pass characteristics or bandwidth of each filter may be somewhat greater than the width of each Zeeman absorption line and the filters are respectively centered on the frequencies (foifn) of those lines. The energies so selected are respectively demodulated by demodnlators 22A and 2.223.

The outputs of the demodulators include, for each sweep cycle of repetition frequency ii, a pair of pulses whose relative amplitude and phase relation each uniquely depend upon the sense of deviation of the frequency ft of the microwave oscillations from the desired frequency in. The pulse output of the demodulators is impressed desired carrier frequency.

upon a network 23, specific forms of which are later herein described, to produce a unidirectional output voltage ec which is zero for null frequency-deviation of the microwave oscillations and which is of polarity dependent upon the sense of the deviations when existent. This error-voltage as may be measured as by a vacuum tube volt meter 25 or equivalent, whereupon an operator may adjust a frequency control 39 of oscillator to correct for the frequency error. Preferably, however, the control voltage as is applied automatically to stabilize the frequency of oscillator 10. it may be applied to a frequency control electrode of the oscillator, as in the case of a klystron or magnetron or it may be applied to a control tube associated with the oscillator.

Assuming the relationships shown in Fig. 2a, the intensity of the Zeeman field is periodically varied at frequency f1 so that the Zeeman lines L1. and LU sweep ranges of frequencies respectively above and below the In the particular arrangement shown in Fig. 1, this variation of the Zeeman field is effected by superimposing an alternating current of frequency fr from oscillator or modulator 20 upon direct current supplied by source 17 to coil 16, the particular coupling element shown in Fig. 1 being a resistor 19 although other coupling arrangements may be used. By way of example, rate frequency f1 may be 1000 cycles per second. For pulse-amplitude comparison, the modulation factor is suitably less than unity so that the highest frequency reached by Zeeman L1. is slightly lower than (fk-fm) and the lowest frequency reached by Zeeman line Lo is slightly higher than (fk-t-fm). When the microwave oscillations are of the desired frequency, the microwave absorption by the gas will be equal in the two swept frequency ranges and the maxima of the two absorptions will occur at the same time in the sweep cycle. When the frequency of the microwave oscillations is above the desired frequency, there is greater absorption of the upper sideband U, and, conversely, when the frequency of the generated oscillations is below the desired frequency, there is greater absorption of the lower sideband L. In other words, the output pulses of one of the demodulators 22A, 22B is greater than the output pulses of the other, depending upon the sense of the frequency-deviation of oscillator 10. This difference in amplitude of the pulses may be utilized to produce a control voltage 8c of polarity dependent upon the sense of deviations by an amplitude-comparator, one form of which is shown in Fig. 4 and later described.

Furthermore (reverting to discussion of Fig. 2a), one or the other of the pulses occurs earlier in the sweep cycle, depending upon whether the frequency-deviation is positive or negative, that is, above or below the desired frequency, and this phase-difference may be utilized to produce a control voltage as of polarity dependent upon frequency-error by a phase-comparator, one form of which is shown in Fig. 5 and later described. When phase-comparison is used, the sweep ranges may be greater than above given for pulse-amplitude comparison.

When, as shown in Fig. 2b, the fixed lower and upper sideband frequencies are respectively lower and higher than the lowest and highest Zeeman line frequencies, a deviation from the desired frequency of the microwave oscillations also produces inequality of the pulse amplitudes and reversal of theirphase relationship, for opposite senses of their frequency-deviation. However, because of the interchange in frequency location of the sidebands and Zeeman lines, the polarity and phase relation of the output voltage 60 is reversed. Specifically, when the frequency of oscillator there is greater absorption of the lower sideband energy; and for subnormal frequency, there is greater absorption of the upper sideband energy. Either of the methods exemplified by Figs. 2a and 2b may be used for automatic stabilization if the aforesaid relationships are appreciated and the control voltage 80 applied in proper 16 is above normal,

4 sense to minimize deviation of oscillator 10 from frequency f0.

To effect sweeping of frequency ranges above and below the desired frequency, the modulation-frequency f1 may be applied to the sidebands (Figs. 3a, 3b) instead of to the Zeeman lines (Figs. 2a and 2b). Specifically, as shown in Fig. 1, the low-modulating frequency it may be applied (as by shifting the position of a switch) periodically to vary the frequency fm of the amplitudemodulator or modulating oscillator 14 either abruptly or continuously between two frequencies. The mean frequencies (fkifm) of the lower and upper sidebands (L, U) may be respectively lower and higher than the fixed frequencies (foifn) of the Zeeman lines LL and Lu (Fig. 3a) or may be between those frequencies (Fig. 3b) as above stated, either relationship may be chosen by choice of the intensity of the Zeeman field and/or of the modulating frequency fm.

In either case, the pulse output of the demodulators 22A and 22B is zero when the frequency of the microwave oscillations corresponds with the desired frequency; but upon deviation therefrom, the difference between the amplitudes of the pulses and their phase relationship contain frequency-error information convertible to an error-voltage by a pulse-amplitude comparator or a phasecomparator. As evident from Fig. 3a, when the carrier frequency of the microwave oscillations shifts upwardly from the desired frequency f0, there is greater absorption of the lower sideband energy; conversely, upon shift from the desired frequency ft, to lower frequency, there is greater absorption of the upper sideband energy. Thus, the algebraic sign of the relative amplitude of the pulses reverses with reversal of the sense of the frequency deviation of the microwave oscillations. When the initial conditions are selected as in Fig. 3b, the absorption of upper sideband energy increases with increase of frequency above normal and absorption of lower sideband energy increases with decrease of frequency fk from normal. Again the algebraic .sign of the relative amplitude of the pulses reverses with reversal of the sense of the deviation of oscillator frequency ft; from its desired value 1%.

In 'Fig. 4, there is shown one of various types of detector circuits suitable for comparing the relative amplitudes of the pulses produced by any of the foregoing methods (Figs. 2a, 2b, 3a, 3b) for production of an error signal of polarity reversing with change in the sense of deviation of the microwave oscillations and of magnitude corresponding with the magnitude of the deviation. Specifically, the demodulators 22A and 22B of comparator 23A maybe pair of diodes 30 connected in seriesopposition in a network including load resistors 32, equal resistors 31, 31 whose common connection provides output terminal of the network. The differential output of the two diodes or equivalent non-linear resistances is applied to an integrating network comprising resistor 33 and condenser 34 connected between the common terminal -45 of resistors 31, 31 and the other output terminal 46 of the network. The value of each of resistors 31 is high compared to that of resistor 33 to minimize interaction between the diodes. The voltage drop across the integrating network 33, 34 is a steady voltage rather than a series of pulses and is of polarity dependent upon whether the output pulses of demodulator 22A are greater or smaller than those of demodulator 22B. This errorvoltage ee, in the case of a reflex klystron, may be applied to control the anode potential of the klystron, or preferably and as shown in Fig. 4, it may be applied to the signal grid of a control tube 36. The resistor 37 is common to the anode circuit of both the control tube and the reflector circuit of the klystron 35 so that the potential of the reflector of the klystron, its frequency-controlling electrode, varies with changes of the output of the amplitude-comparator network 23A so to effect automatic frequency stabilization of the microwave oscillations. The operating frequency of the klystron may be initially set, or readjusted, by the potentiometer network 38, 39 in the grid circuit of the control tube or may be set, or readjusted, by adjustment of cavity dimensions of the klystron.

A suitable type of phase-comparator network for use in the system of Fig. 1 and for comparison of the phase of pulses produced by any of the methods of Figs. 2a, 2b, 3a, 3b is shown in Fig. 5: other suitable phase-comparator arrangements are shown in copending application Serial No. 148,481, filed March 8, 1950. In the particular arrangement shown in Fig. 5, a pair of rectifiers, such as diodes 40, 40, are connected in series in a direct-current loop including a pair of resistors 41, 41, the errorvoltage being produced between the common terminal 45 of the resistors and the terminal 46 common to the cathode of one of the diodes and the anode of the other. The output pulses of one of the demodulators 22A, 22B are each converted to a pair of pulses A1, A2 of opposite polarity which are respectively applied through condensers 42, 42 to the anode of one rectifier and the cathode of the other. This conversion may be effected as shown in Fig. 5 by using two rectifiers A1, MA; in the demodulator itself; or as shown in copending application including Serial No. 29,836, now abandoned, a pushpull stage may be interposed between the phase-comparator network and a demodulator having a single rectifier.

The output pulses B of the other demodulator, specifically 22B of Fig. 5, are applied across resistor 44 connected between the common terminal of resistors 43, 43 and a ground or other return conductor. These pulses are thus applied in push-push to the anode of one rectifier 4th and the cathode of the other. Thus, which of the rectifiers 40, 44) is conducting in a cycle of the sweep or repetition frequency f1 depends upon the phase relation between the pulses impressed upon the input circuits of the phase-comparator, which in turn as above described, depends upon the sense of the deviation of frequency fl; from the desired frequency f0.

The output pulses of comparator-network 23B are impressed upon its integrating circuit 33, 34, 34A so to produce a steady error-voltage of polarity reversing with reversal of the phase relations between the pulses in the two channels of the control system. This control voltage e0 may be applied, as above described, automatically to stabilize the frequency of the microwave oscillations. Specifically, as shown both in Figs. 4 and 5, the control voltage may be applied to the signal grid of a control tube 36 associated with a klystron oscillator. The input or output connections of the comparator-network 23B must be reversed, and the same is true of comparator-network 23A, if there is transition from operation in accordance with Fig. 2a (or 311) to operation in accordance with Fig. 3a (or 317).

It shall be understood the invention is not limited to the arrangements specifically illustrated and described and that changes and modifications may be made within the scope of the appended claims.

What is claimed is:

l. A system of producing an error-signal corresponding in sense with deviations of the carrier frequency of microwave oscillations which comprises means for modulating the oscillations to produce a pair of sidebands at frequencies respectively lower and higher than the carrier frequency, means for impressing the modulated oscillations upon gas having a sharp molecular absorption line, means for producing a Zeeman field in said gas to split said absorption line into a pair of absorption lines at frequencies respectively adjacent the sideband frequencies and respectively above and below the desired carrier frequency, means for cyclically sweeping one pair of said pairs over ranges of frequency including the frequencies of the other pair, means for selectively demodulating the microwave energy transmitted by the gas in said ranges of frequency to produce a pair of pulses for each sweep cycle,

and means for combining said quency-error signal of reversible the sense of deviation of said carrier frequency.

2. A system as in claim 1 in which the Zeeman field is modulated at low frequency to effect cyclic sweeping of fixed frequency sidebands by the pair of varying frequency absorption lines.

3. A system as in claim 1 in which the sidebands are modulated at low frequency to effect cyclic sweeping of a pair of absorption lines of fixed frequencies by varying frequency sidebands.

4. A system as in claim 1 in which the polarity of the error-signal is dependent upon the relative amplitude of the paired pulses.

5. A system as in claim 1 in which the polarity of the error signal is dependent upon the time relation of the paired pulses in the sweep cycle.

6. A system as in claim 1 in which the error signal is applied to vary the potential of a frequency-control electrode of a tube for stabilization of the carrier frequency of the microwave oscillations.

7. A system as in claim 1 in which the ranges of frequency are respectively higher and lower than the desired carrier frequency and exclusive of it.

8. A system as in claim 2 in which the frequency of the lower sideband is higher than the mean frequency of the adjacent sweeping absorption line and the frequency of the upper sideband is lower than the mean frequency of the adjacent sweeping absorption line.

9. A system as in claim 2 in which the frequency of the lower sideband is lower than the mean frequency of the adjacent sweeping absorption line and the frequency of the upper sideband is higher than the mean frequency of the adjacent sweeping absorption line.

10. A system as in claim 9 in which the frequency ranges of the sweeping absorption lines are mutually exclusive and exclusive of the desired carrier frequency.

11. A system as in claim 3 in which the mean frequency of the lower sweeping sideband is lower than the fixed frequency of the Zeeman absorption line adjacent thereto and the mean frequency of the upper sweeping sideband is higher than the fixed frequency of the Zeeman absorption line adjacent thereto.

12. A system as in claim 3 in which the mean frequency of the lower sweeping sideband is higher than the fixed frequency of the Zeeman absorption line adjacent therepulses toproduce a frepolarity dependent upon absorption line adjacent thereto.

13. A system as in claim 12 in which the frequency ranges of the sweeping sidebands are mutually exclusive.

14. A system for producing an error-signal corresponding in sense with deviations of the carrier frequency of microwave oscillations which comprises means for modulating the microwave oscillations to produce sidebands at frequencies respectively higher and lower than said carrier frequency, an enclosed body of gas upon which the modulated oscillations are impressed and which has absorption line, magnetic means for sideband frequencies and carrier frequency, sweep-frequency modulating means for effecting relative sweeping of the paired sidebands and the paired absorption lines in frequency ranges respectively above and below the desired frequency of said oscillations, means for selectively demodulating the microwave energy transmitted by said gas in. said ranges of frequency to produce paired pulses during successive cycles of said sweep-frequency, and a detector network upon which said paired pulses are impressed. to produce an error signal.

15. A system as in claim 14 in which the sweep-frequency modulating means varies the intensity of the Zeeman field cyclically to vary the frequencies of the paired absorption lines over said frequency ranges.

1-6. A system as in claim 14 in which the sweep-frequency modulating means cyclically varies the upper and lower sidebands over said frequency ranges.

17. A system as in claim 14 in which the detector network is of amplitude-comparator type to produce a unidirectional output voltage of polarity and magnitude dependent upon the relative amplitude of the paired pulses.

18. A system as in claim 14 in which the detector network is of phase-comparator type to produce a unidirectional output voltage of polarity and magnitude dependent upon the time relation of the paired pulses in the sweep cycle,

References Cited in the file of this patent UNI-T ED STATES PATENTS White June 30, 1942 Hershberger r rMay 29, 1951 OTHER REFERENCES The Zeeman Effect in Microwave Molecular Spectra, by Jen in the Physical Review for November 15, 1948, pages 1396 to 1406. 

